1. Field of the Invention
The present invention relates to a noise estimator device of a video decoder and related method, and more particularly, to a noise estimator device and related method capable of detecting durations of synchronization signals and determining noise levels according to the detected durations of the synchronization signals.
2. Description of the Prior Art
With the rapid development of the communication and computer technologies, video applications are more and more various. Most video applications, such as video conferences, video phones of the public switched telephone network or the mobile phone network, web cameras, and broadcast analog or digital TV, need efficient decoding operations, such that video signals can be reconstructed to video data for showing images with video output devices.
During the signal transmission (especially through radio channels), radio signals are inevitably interfered by noises, landforms, or terrain features, so that the transmitted signals are mixed with the noises, which cannot be analyzed. Therefore, the recognition ratio of the signals received by a reception end is very low, i.e. a corresponding ‘eye’ range is very small when the received signals are observed with an eye diagram. In this case, in order to decode broadcasted video signals correctly, the prior art video decoder usually applies a noise estimation device for estimating a noise level, so as to enhance the decoding reliability. Please refer to FIG. 1. FIG. 1 is a schematic diagram of a prior art video decoder device 10. The video decoder 10 includes an analog to digital converter 100, a digital signal processor 102, and a noise estimation unit 104. The analog to digital converter 100 is utilized for sampling and transforming analog video signals into digital signals. The digital signal processor 102 is utilized for performing calculations (such as demodulation or decoding) of signals outputted from the analog to digital converter 100, and outputting digital video signals to a video output device. In general, a video signal includes a horizontal or vertical synchronization signal Hsync/Vsync for defining a timing sequence of video data. Detecting a front porch period and a back porch period of the horizontal synchronization signal Hsync, the digital signal processor 102 can detect durations of the horizontal synchronization signal Hsync. The noise estimation unit 104 can determine the noise level of the analog video signals according to the durations of the horizontal synchronization signal Hsync, so that the digital signal processor 102 can compensate influence of noises.
In order to estimate the noise level accurately, it is important to determine the durations of the horizontal synchronization signal Hsync. In the prior art, the decision level for determining the durations of the horizontal synchronization signal Hsync is obtained by calculating a signal average value of each preset number of signal periods, counting the number of the signal periods that the signal average value is calculated, and then determining whether the signal average value is larger than a preset value and whether the number of the signal periods being counted is equal to half a number of the horizontal scan lines. If the signal average value is smaller than the preset value or the number of the signal periods being counted is equal to half the number of the horizontal scan lines, the preset value is set equal to the decision level of the horizontal synchronization signal. Conversely, when the number of times of this case (the average value is smaller than the preset value or the number of the signal periods being counted is equal to half the number of the horizontal scan lines) is greater than a predetermined value, the decision level of the horizontal synchronization signal is set as half of the sum of the preset value and the average value.
Since the prior art calculates the signal average value of each preset number of periods, the decision level of the horizontal synchronization signal Hsync is not reliable when signal-to-noise ratio (SNR) is getting worse. Hence, the determined durations of the horizontal synchronization signal Hsync are not correct, so that the noise level cannot be currently determined. In this case, the digital signal processor 102 cannot eliminate the effect of noises. Therefore, the quality of the digital video signal is poor.